The invention relates to devices for holding wafer-shaped articles, such as semiconductor wafers.
Semiconductor wafers are subjected to various surface treatment processes such as etching, cleaning, polishing and material deposition. To accommodate such processes, a single wafer may be supported in relation to one or more treatment fluid nozzles by a chuck associated with a rotatable carrier, as is described for example in U.S. Pat. Nos. 4,903,717 and 5,513,668.
The patents referenced above operate on the Bernoulli principle, such that the wafer receives subjacent support from a gas cushion rather than by contact with the chuck. Such chucks nevertheless ordinarily include a circular series of pins that are positioned radially outwardly of a wafer positioned on the chuck. Those pins prevent lateral displacement of the wafer relative to the chuck.
As described in U.S. Pat. No. 4,903,717, it is also advantageous to construct the pins such that each projects upwardly from a respective pivotal base. The axes of the pin and base are vertical but offset from one another such that pivoting of the base causes the associated pin to travel along a circular arc and hence be adjustable in its radial position. The pivot bases are each provided with gear teeth, which mesh with the teeth of a common gear ring that is coaxial with the axis of rotation of the chuck. Rotation of the gear ring relative to the chuck thus causes all of the pins to move conjointly and to the same extent.
That construction permits the pins to be moved radially outwardly for placement or removal of a wafer, and then to be moved radially inwardly to be brought into contact with the peripheral edge of a wafer. Such contact prevents not only lateral displacement of the wafer relative to the chuck, but also relative rotation between the wafer and chuck as the chuck is spun.
Rotation of the wafer relative to the chuck is referred to as “wafer shift.” Whereas the wafer and chuck are generally not intended to move relative to one another, a controlled amount of wafer shift is desirable to ensure correct cleaning and/or etching of the wafer bevel in the area where the pins touch wafer. This involves shifting the wafer by several degrees relative to the chuck during the cleaning as well as etching step. The wafer shift is carried out in a specific time interval. Within this time interval the gear ring inside of the chuck opens the pins of the chuck and the wafer is shifted. The gear ring is moved back by spring forces.
In spin chucks utilized in process modules for single wafer wet processing, such as those made by Lam Research Corporation, wafer shift can be performed by accelerating or decelerating the chuck during the process steps. That is, the inertia of the gear ring, which drives the chuck pins, is such that there is slight relative rotation between the gear ring and the chuck body during acceleration and deceleration of the chuck, which causes the pins to undergo an opening and closing movement and release the wafer for several milliseconds as the chuck is accelerated or decelerated. During that interval the wafer is not clamped and rotates by several degrees relative to the chuck. Similarly, if the tooth gear is driven instead of the chuck base body the relative movement of the chuck base body against the tooth gear is caused by the inertia of the chuck base body, with the same effect on the chuck pins.